i`m installing a magnum XL-52 4 stroke in my cmp .40 -50 p-51 mustang
i wish to install a 4 bladed prop for scale looks and performance.
this is my first foray into 4 strokes, and need to know what size prop to install
any help or tips would be most appreciated
thanks in advance

You may get scale "looks" with a 4 blade prop but odds are you will get very un scale-like performance . The 4 blade is a real performance killer in most cases.
Try a Xoar 12x6 or 11x7 for starters is my best guess . Having never owned the Magnum however, that guess may be a bit off .

Zinger 4 blade props work well. I used one on a PTS mustang. 10x8x4 on the Evo .45 trainer engine. Looked nice after painting it black with yellow tips.
It just didnt seem to have the peak performance of the 2 bladed prop.
But wasnt enought of a difference to upset me.
It flew well.

Your 4 stroke will have a bit more torque so it should handle a 11 or 12 in prop nicely, but at 25 dollars for the Zinger prop be sure of the one you want.
And then the 4 bladed spinner. I had one cut for the prop at Truturn. Another 35 or 40 dollars. Been a while, so dont quote me on that.

I still have it. Its still pretty

If you go the Zinger route, be sure to order extra blades.
The Zinger prop is wood, comes balanced. 4 wooden blades mounted in an aluminium disk.
The blades are replacable. It will NOT stand a ground strike.

APC makes some nylon 4 blade props, but they dont have the "look".

I would go with a 2 blade prop for flying, and make a nice scale prop for static, anything else is going to be a compromise, as the size of 4 blade prop you are going to turn won't look particularly scale, as it will be very much undersized in comparison with the "scale" diameter.

Going to a 4-blade prop won't do anything positive for performance, the full size prop is 4 bladed mainly because a 2 bladed prop of the right size would be too big for the landing gear, and potentially generate too much torque and tip speed. In most cases, a 2 bladed prop is the most efficient, we don't have to worry about it being too big, as generally, model props are a lot smaller in relation to the airframe than full size ones.

now this is interesting people are saying to use the 2 blade prop,
1, the "scale" diameter. is a valid argument
2 real performance killer in most cases, now I have read somewhere that a bench test of a multiple blade prop it will give a false reading. because when the plane is flying the prop always has fresh air and the prop turbulence is reduced, when the plane is on the bench with multiple blade props it will give a performance reduction reading because of prop turbulence.
again this was not my test I read this somewhere I cannot remember, I would like to know if it is true

I don't know the answer to the performance issue, I'm following the herd somewhat, but remember that the blades aren't always in clean air, as the prop isn't 100% efficient like a screw driving through wood, it slips by quite a large amount.

Remember also, that it was (or is?) reasonably common for speed models (especially control line) and rubber duration models to use single bladed props with a counterbalance. Again, there has to be a performance gain from going to a single bladed prop.

Where that gain comes from and whether it can be quantified, is, however, beyond my limited brain

You may get a little more thrust out of the right 4 blade . This will usually come at the expense of flat out speed compared to a 2 blade. At least that was what I was trying to get across regarding the performance issue in post 2.

One way to find out is to try. Been there done that. Let us know what you discover.